1. Field of the Invention
The invention relates to a power transmitter for a battery system, a battery system having such a power transmitter, and a motor vehicle having such a battery system.
2. Description of the Prior Art
There is a growing need for battery systems that are to be used in stationary applications, such as wind farms and emergency current systems, but also in vehicles. All of these requirements make stringent demands in terms of reliability and fail-safety. The reason for this is that a complete failure of the voltage supply by the battery system can cause a failure of the entire system. In wind farms, for instance, batteries are used for adjusting the rotor blades in strong winds and protecting the system in this way against excessive mechanical stresses that can damage or even destroy the wind farm. In the case of battery failure in an electric car, the car could not be driven. An emergency current system, in turn, should specifically ensure uninterrupted operation, for instance of a hospital, and therefore must itself as much as possible be incapable of failing.
To make it possible to furnish the requisite power and energy for the particular application, individual battery cells are connected in series and sometimes additionally in parallel. FIG. 1 shows a basic circuit diagram for a series circuit of batteries. Many battery cells 10-1 through 10-n are connected in series, in order to achieve the requisite high operating voltage, for instance in a passenger car for the electric motor, by adding together the voltage of the individual cells 10-1, . . . , 10-n. The high operating voltage can be decoupled by switches 11-1 and 11-2 on the output side from the following power electronics elements, not shown, such as inverters. Since the entire output current of the battery, because of the series connection of the battery cells 10-1, . . . , 10-n, flows in each of the battery cells 10-1, . . . , 10-n, and the charge transport takes place by means of electrochemical processes inside the battery cells 10-1, . . . , 10-n, the failure of a single battery cell in an extreme case means that the entire arrangement can no longer furnish any current and hence no electrical energy. To enable detecting a threatening failure of one battery cell 10-1, . . . , 10-n in good time, conventionally a so-called battery management system 12 is employed, which is connected or connectable to both poles of each of the battery cells 10-1, . . . , 10-n and which at regular or selectable intervals determines operating parameters, such as voltage and temperature, of each battery cell 10-1, . . . , 10-n and from that the state of charge (SoC) of each of them. This entails major expense and at the same time makes for poor flexibility of the electrical operating data of the battery system.
Further disadvantages of the series connection of many battery cells are as follows:
1. For different operating states of the apparatus to be operated with the battery, conditions for the operating voltage to be furnished, the maximum current, and the stored energy, are made that can be combined only if a greater number of battery cells is coupled together than would actually be necessary to meet the requirements. This increases the price and, particularly in an electric car, it also increases the problematic weight and volume of the battery system.
2. Installing the battery, or in other words connecting the individual cells together, is done because of the voltages, added together by the series connection, of the individual battery cells, which at high voltages are up to 1000 V; therefore, replacement of the battery or individual cells or modules cannot be done in local facilities, or in the case of a stationary application, it can be done only by especially trained professionals using special tools. As a result, the logistical effort and expense for the maintenance of battery systems in the event of a malfunction is high.
3. To switch the battery system without voltage, or in other words to disconnect the actual battery from the load, power switches 11-1 and 11-2 must be provided, which are typically embodied as contactors and are very expensive, given the high currents and voltages to be expected.